Computer randomly shutting down

[Arkwolf]

So my specs are as follows, anything slashed out has been changed since it started.
CPU: i7 2600k

Mobo: Asus P8Z77I DELUXE
GPU: GTX 780ti, GTX 690, GTX 560ti
PSU: Coolermaster 650watt bronze, Corsair 750watt bronze
HDD: 1TB WD Green

So basically this all started with random screen lockups, the screen would lock up in game. I was still able to alt tab and sometimes and then it would say display drivers have crashed. So I updated EVERYTHING (Bios, drivers etc etc). I then purchased a better power supply since my coolermaster was 6 years old.However screen lockups still occurred. So I decided to switch out my GTX 690 and all the problems went away.

 

So I came to the conclusion it was GPU so it got sent away to get fixed. However I wanted my 690 back into my main pc so I put in a 560ti into this computer. Then all of a sudden the computer only under load will instantly shutdown, it will then try and reboot with only about 30% success rate. So then I thought overheating issues so I loaded up speedfan along with heavens benchmark and with nothing maxing over 60c it then just shutsdown. There is no bios warnings such as overclock (which it isnt), power surge or anything else. I tried to look through event viewer and can only find "the computer unexpectedly shut down" and no other signs of drivers crashing or anything. 

Tomorrow I will be trying a fresh OS along with ANOTHER nvidia GPU, however I would like to try and keep this PC on the existing hdd as it has a lot of paid software that I would hate to lose it. Suggestions are much appreciated. 

[nanowatt]

Did you switch the ram?Try using 1 sick at a time maybe one is broken.Since is shutting down only under load there are 2 things that may cause it voltage or heat.If you did any oc please return to stock speeds also run the benchmark i need the exact temp on mb and cpu

[Arkwolf]

Haven't considered testing ram mainly cause it ran fine on my 690 with no problems. However I will add that onto the list after the previous 2 things.

[Arkwolf]

Sorry to dig this back up but I thought since we finally sorted it like 2 months later I should post what was wrong. First was during the GPU swap to the 560ti I knocked out the molex cable to the fan controller which caused it to overheat. Also the 780ti Palit GPU was faulty, we discovered this after a little longer when funny things started happening. Since then I had the 780ti replaced and got a new one which then died 1 month later which is currently getting RMA'ed again -.-.